It is known to provide optical wireless data communications by using visible light (or infrared or ultraviolet light) instead of radio frequencies to transmit and receive data wirelessly between devices. Data may be transmitted in visible light by modulating the intensity of the light. The light used may be coherent or incoherent. Optical wireless networks using visible light may in some circumstances allow a higher data capacity and greater energy efficiency than radio frequency wireless networks.
Carrier sensing multiple access with collision avoidance (CSMA/CA) is a technique that is employed in wireless communication standards such as 802.11 and 802.15.4 to allow sharing of a common transmission medium between multiple entities in a given network. The use of CSMA/CA may provide a mechanism for minimising the probability of simultaneous transmissions in the wireless channel. The occurrence of simultaneous transmissions by two or more devices may be called a collision. Such collisions may adversely affect the overall throughput of the system.
The basic operating principle for CSMA/CA may be described as follows.
At a first stage, before transmitting a message (for example, a frame) to a receiving node, a device that wishes to transmit senses the channel in order to determine whether the channel is free or busy. Sensing the channel may comprise any method of detecting whether any signal is currently being sent on that channel. The sensing of the channel may be performed using simple energy detection or using another method, for example using a more sophisticated signature detection mechanism.
At a next stage, if the device detects that the channel is busy, the device waits for a random back-off period and then senses the channel again to determine whether the channel is still busy. The channel may be busy if any other device (which may be the receiving node or any other node of the network) is currently transmitting using that channel.
At a further stage, if the device detects that the channel is free, the device commences transmission of its message. If the transmission of the message is successful, an acknowledgement (ACK) is sent by the receiving node to the device. If the device does not receive an ACK from the receiving node, the device waits for a random back-off interval and then senses the channel again to determine whether the channel is busy. If the channel is not busy, the device attempts to re-transmit the message.
A central premise of the CSMA/CA technique is that each node is able to sense the transmission of every other node in a given wireless network. Therefore, when a given device senses the channel on which it wishes to transmit, it can tell whether any other device is currently transmitting using that channel.
The CSMA/CA technique may be considered to be suited to the half-duplex nature of some radio-frequency (RF) wireless systems. In some radio-frequency (RF wireless systems), the same frequency is utilized for uplink and for downlink. Such wireless systems may operate in a half-duplex manner in which the same channel is used for transmitting and receiving. A node may either be receiving (for example, channel sensing) or transmitting. In a half-duplex RF wireless system a node may never be both transmitting and receiving at the same time.
A requirement of CSMA/CA may be that any given device can sense the channel across the whole system (i.e., can sense transmissions from any node in the network). In a real-world wireless network, the requirement of being able to sense the channel across the whole system may be difficult to satisfy due to the hidden node issue. The hidden node issue is illustrated in FIG. 1. The hidden node issue occurs when a first device in a network is unable to sense transmissions from a further device in the network. The first device may believe that the channel is free when in fact the further device is transmitting. Therefore, collisions may occur.
When the hidden node issue is present, additional measures may be required to be taken to promote collision-free channel access.
FIG. 1 is a schematic diagram which is illustrative of a wireless network comprising an access point (AP) 10 and two stations (STA) 12, 14. The wireless network of FIG. 1 is subject to the hidden node issue. Station 12 cannot sense transmissions from station 14, and station 14 cannot sense transmissions from station 12.
Circle 16 is representative of the transmission range of station 12. Station 14 is not within the transmission range of station 12 and so cannot sense transmissions from station 12.
Circle 18 is representative of the transmission range of station 14. Station 12 is not within the transmission range of station 14 and so cannot sense transmissions from station 14.
The access point 10 is within the transmission range of both stations. Standard CSMA/CA may not work in the wireless network illustrated in FIG. 1 because collisions may occur at the access point 10 when both stations 12, 14 attempt transmission simultaneously.
In the 802.11 standard, the distributed coordination function (DCF) protocol may address the hidden node issue. There are two methods for packet transmission in DCF.
The DCF protocol provides a first packet transmission method comprising a two-way handshaking mechanism (also called the basic access method). In the two-way handshaking mechanism, nodes utilise standard CSMA/CA without any regard for the hidden node issue. When utilizing the DCF protocol with two-way handshaking, receiving nodes transmit a positive acknowledgement (ACK) to signal a successful reception of a transmission.
The DCF protocol provides a second packet transmission method comprising a four-way handshaking mechanism (RTS/CTS) in which nodes must reserve the channel before any transmission. When using the four-way handshaking mechanism for transmission, any node wishing to access the channel must transmit a short ready-to-send (RTS) message to the intended receiver. If the RTS message is received successfully by the receiver, the receiver will transmit a clear-to-send (CTS) message to the node to indicate that the channel has been reserved. The RTS/CTS exchange is then followed by transmission of a data frame from the transmitting node to the receiver, and transmission of an ACK from the receiver to the transmitting node.
Although the four-way handshaking scheme may in some circumstances overcome the hidden node issue, the use of the four-way handshaking scheme may introduce a significant overhead. The time taken to send data may be increased by the requirement to send RTS and CTS messages. The use of the four-way handshaking scheme may lead to a reduction in the overall throughput of the system (for example, compared to a system that uses two-way handshaking).
In order to provide multiple access functionality in an optical wireless system, one may wish to utilise the DCF mechanism from the 802.11 standard. However, certain characteristics of some optical wireless nodes may prevent stations from sensing each other. If stations cannot sense each other, the basic access method (two-way handshaking) may be rendered inoperable. Only the four-way handshaking mechanism of 802.11 and not the two-way handshaking option may be used.
The characteristics of optical wireless nodes that may prevent stations from sensing each other may include directionality and/or wavelength division.
With regard to directionality, an optical wireless station (STA) may have a transmission cone with a half angle less than 90 degrees and/or a receiver field-of-view of less than 180 degrees. As a result, one optical wireless station may be unable to detect transmission from another optical wireless station, even if the optical wireless stations are placed side-by-side.
With regard to wavelength division, a given optical wireless transceiver may use a different wavelength of light for uplink than is used for downlink. For example, a STA may be equipped with an infrared emitter (for example, 850 nm) and a blue light receiver (for example, 460 nm). As a result, a given STA may not be able to detect a transmission from another STA even if the STAs are positioned face-to-face. Because all the STAs are configured to transmit on a first wavelength and receive on a second, different wavelength, a STA cannot receive another STA's transmission. A STA can only receive a transmission from a device that transmits on the second wavelength (for example, an AP), not a transmission from a device that transmits on the first wavelength.